Impact of Seaweed Extracts, Protein Hydrolysates, and a Fertilizer Containing 5-Aminolevulinic Acid on Yield, Quality, and Nutraceutical Traits of Annurca Apple at Harvest and During Cold Storage

Annurca’ apple is a traditional Italian cultivar, belonging to the germplasm of the Campania Region, appreciated for its distinctive sensory quality, flesh firmness, and characteristic postharvest reddening during the “melaio” stage. Biostimulants are increasingly used in apple production to improve fruit performance and postharvest quality while reducing conventional inputs. This study evaluated the effects of two biostimulant-based treatments and a 5-aminolevulinic acid (5-ALA)-based formulation, on yield, fruit growth, commercial and organoleptic quality, and nutraceutical traits of ‘Annurca’ apple at harvest and during cold storage. Treatments SE (seaweed extract mix) and APH (animal protein hydrolysate) increased yield efficiency by about 26% compared with the control, while treatment 5-ALA produced the largest fruits at harvest (+9.9%), followed by APH and SE (+5.5%), resulting in a greater percentage of fruits in the highest commercial size classes. Treated fruits also showed reduced weight loss during storage, particularly with treatment 5-ALA, and higher dry matter content at harvest. Most postharvest quality traits were influenced primarily by storage duration rather than by treatments. Total phenolic content remained consistently higher in the peel than in the pulp throughout storage, confirming the peel as the main reservoir of bioactive compounds, whereas biostimulant effects on antioxidant capacity and phenolic content were limited and treatment-dependent. Similarly, procyanidin B2 was consistently more abundant in the peel than in the pulp throughout cold storage, confirming its preferential accumulation in the outer fruit tissues. Among the treatments, the 5-ALA-based formulation better preserved procyanidin B2 levels during storage, highlighting its potential to maintain the nutraceutical quality of the fruit. Overall, biostimulants, particularly treatment 5-ALA, improved yield, fruit size, and storage performance without altering the typical quality profile of ‘Annurca’ apple.